1. Field of the Invention
The present invention relates to film coating of a mold core for glass molding, and particularly relates to a method and device for making high precision hard film coating of a mold core for glass molding.
2. Description of Prior Art
Hard film coating of a mold core is one of the most important factors contributing to glass molding success. The hard film coating has a determinative influence on the surface quality of the resultant optical glass lens. Further, the life expectancy of the mold core is also influenced by the hard film coating process, which directly affects the production cost.
Currently, PVD (Physical Vapor Deposition) processes, such as evaporation, sputtering and ion plating, are commonly applied to efficiently form a hard film coating on the outer surface of the mold core.
In the film coating process, mold jigs are generally employed to increase the coating efficiency and productivity. However, some disadvantages are presented when using conventional mold jigs in film coating processes. These disadvantages include large eccentricity of the mold jig, peeling or cracks of the resultant hard film from the mold core, poor molding process regularity, insufficient adhering strength of the hard film relative to the mold core, poor precision of the external diameter of the coated mold core, and difficulties in precision machining of the coated mold core.
FIG. 1 shows a conventional mold jig 100 for use in the hard film coating process of a mold core for glass molding. The mold jig 100 is in a planar, rectangular or circular shape, and has a first surface 102 and an opposite second surface 104. For facilitating description, the first surface 102 and the second surface 104 are hereinafter referred to as the upper surface and the lower surface, respectively. However, it should be understandable that these direction related terms are for illustrative purposes only, and are not restrictive.
A plurality of through holes 106 (only one shown for simplification) is defined between the upper surface 102 and the lower surface 104 of the mold jig 100. For facilitating description, this direction, i.e., the direction between the upper surface 102 and the lower surface 104, is hereinafter referred to as an axial direction. The direction perpendicular to this axial direction, or parallel to the upper surface 102 and the lower surface 104, is hereinafter referred to as a radial direction.
Each through hole 106 receives one mold core 108 having a molding surface with a hard film 110 deposited thereon by PVD process. The mold core 108 includes a body 112 firmly received in the through hole 106 and extending in the axial direction. The body 112 projects beyond the upper surface 102 of the mold jig 100, and its exposed upper surface forms the molding surface of the mold core 108.
The thickness of the planar mold jig 100, i.e., the dimension between the upper surface 102 and the lower surface 104, is smaller than the thickness of the mold core 108, i.e., the axial dimension of the mold core 108. Therefore, the molding surface of the mold core 108 projects beyond the upper surface 102 of the mold jig 100 when the body 112 of the mold core 108 is received in the through hole 106.
The mold core 108 further includes a base 114 connected with the body 112. The base 114 has an outwardly projecting flange 116, whereby a step portion is formed together with the body 112 to hold the mold core 108 on the lower surface 104 of the mold jig 100. An end stop 118 presses on the base 114 to further hold the mold core 108 in the through hole 106.
As the body 112 of the mold core 108 projects beyond the upper surface 102 of the mold jig 100, the hard film 110 will be deposited on both the exposed molding surface and side surface of the mold core 108 and thus has a sufficient adhering strength relative to the body 112. However, since the body 112 of the mold core 108 projects beyond the upper surface 102 of the mold jig 100 to a great extent, a large portion of the side surface of the body 112 will be exposed for deposition, whereby a thick and irregular side film 120 will be inevitably formed on the exposed side surface portion of the body 112.
As shown in FIG. 2, in the glass molding process applying this mold core 108, when the thus coated mold core 108 is coreed into a mold cavity 902 of the mold 900, the thick and irregular side film 120 on the body 112 will bring the eccentric or tilting problem. That is, the axial line 122 of the mold core 108 will be offset from, or tilted relative to the axial line 904 of the mold cavity 902. In some serious cases, even the base 114 of the mold core 108 and the clamp 906 cannot correct this eccentricity or tilting. Therefore, this mold core 108 cannot be applied in molding processes requiring for high centricity. When applied, in the following assembling or machining processes, the irregular side film 120 will also tend to peel or crack from the mold core 108, which has a bad impact on the yield of the glass molding process.
Further, since the mold core 108 projects too much relative to the mold jig 100, the process regularity of the PVD process and thus the quality of the coated film will also be adversely affected.
FIG. 3 shows another conventional mold jig 100a for use in the hard film coating process of a mold core for glass molding. Similarly, a through hole 106a is defined between upper and lower surfaces 102a, 104a of the mold jig 100a for receiving a mold core 108a therein. In this technology, the height of the body 112 of the mold core 108a is substantially equal to the thickness of the mold jig 100a, and thus the upper molding surface of the mold core 108a is substantially flush with the upper surface 102a of the mold jig 100a. A chamfer 124 is defined between the upper surface 102a of the mold jig 100aand the inner side of the through hole 106a to expose an upper portion of the side surface of the body 112 of the mold core 108a. Thus, in coating process, a side film 120a will be deposited on this exposed upper portion of the side surface of the body 112 except for the upper film of the resultant hard film 110. The extending depth of the side film 120a along the axial direction of the mold core 108a is restricted by the depth of the chamfer 124, and thus the extending depth of this side film 120a is smaller than that of the side film 120 in FIGS. 1 and 2.
The above coating technology ensures the adhering strength of the resultant hard film 110 relative to the mold core 108a, since the hard film 110 is coated on both upper and side surfaces of the mold core 108a. However, the eccentricity or tilting problem may still be present in the following molding processes, although it is mitigated relative to that of the technology in FIGS. 1 and 2.
Additionally, although the upper molding surface of the mold core 108a is flush with the upper surface 102a of the mold jig 100a so that the regularity of the PVD process is improved, the coated side film 120a may still adversely affect the yield of the molding process due to the eccentricity or tilting problem.
FIG. 4 shows a further conventional mold jig 100b for use in the hard film coating process of a mold core for glass molding. Similarly, a through hole 106b is defined between upper and lower surfaces 102b, 104b of the mold jig 100b for receiving a mold core 108b therein. In this technology, the height of the body 112 of the mold core 108b is substantially equal to the thickness of the mold jig 100b, and thus the upper molding surface of the mold core 108b is substantially flush with the upper surface 102b of the mold jig 100b. What is different from the technology as shown in FIG. 3 is that no chamfer is defined in the mold jig 100b. Therefore, when assembled, the upper molding surface of the mold core 108b is continuous with the upper surface 102b of the mold jig 100b to form a continuous plane. A continuous hard film 110 thus can be deposited on this continuous plane by PVD process. This coating method ensures that the hard film 110 will be deposited on the upper molding surface of the mold core 108b and no hard film will be deposited on the side surface of the body 112 of the mold core 108b. Accordingly, the regularity of the PVD process can be improved to obtain the relatively high quality hard film 110. However, since no hard film is coated on the side surface of the body 112 of the mold core 108b, the adhering strength of the obtained hard film 110 relative to the mold core 108b is diminished. This may result in peeling or cracks of the obtained hard film 110 from the mold core 108b in the following mold assembling or glass molding processes, which has a bad impact on the yield of the glass molding process.
Clearly, none of the above mentioned prior art technologies can ensure both the precision of the hard film coating on the mold core and the sufficient adhering strength of the hard film.
Therefore, to overcome these problems encountered by the prior art, a method and device are required for making hard film coating of high precision and sufficient adhering strength on a mold core for glass molding.